Direct reduction processes have come into increasing prominence in recent years for several reasons. They can be economically built to smaller capacities than blast furnaces. They are better adapted to intermittent operation than blast furnaces. They are more versatile in fuels than blast furnaces, not requiring the high priced coke which is necessary for the latter. Suitable coking coals are not widely distributed and are becoming increasingly difficult and expensive to obtain.
The direct reduction process is primarily a reduction by gases, usually hydrogen and carbon monoxide. In the most widely used processes, according to the literature, a mixture of these gases is produced by "reforming" natural gas by reaction with steam and oxygen. In some cases other hydrocarbon gases or light petroleum fractions have been used.
As is well known, however, natural gas is becoming increasingly expensive and future supplies are in doubt. There is, therefore, an incentive to use other fuels.
One commercial or developmental process utilizes coal as the source of reducing gases. A mixture of coal and iron ore is introduced into the upper end of a huge inclined rotary kiln. Air is introduced at various points along the kiln, producing partial combustion of the coal, elevating the temperature and producing carbon monoxide, which acts as the reductant.
In another proposed process, coal is to be gasified by reaction with oxygen and steam, producing a mixture of carbon monoxide and hydrogen, which is to be used as the reducing gas. In that process the gasification of the coal takes place in either a dilute phase or fluidized bed gasifier and the gas is utilized in a separate reduction furnace.
An extremely large number of United States patents have been granted on direct reduction processes. Therefore, only those which appear most pertinent to this invention will be discussed.
Cavanaugh U.S. Pat. No. 3,427,013 is directed to a "low temperature blast furnace". A mixture of ore with coal, coke, or lignite is introduced into the top of a shaft furnace. Heated air, in a quantity sufficient to cause only partial combustion of the fuel, is introduced at several points in the upper portion of the furnace, producing carbon monoxide, which is considerably diluted by the nitrogen of the air. The ore is successively heated by the combustion of the fuel and reduced by the carbon monoxide. In the lowermost section of the shaft the "metallized ore" is cooled by indirect heat exchange with the incoming air. The furnace operates under super atmospheric pressure, with waste gases being withdrawn primarily from the top of the furnace. Some, however, flows downwardly and out the bottom, serving as an air seal.
This patent does not disclose operation under conditions such as to produce hydrogen, which is recognized as a more effective reducing agent than carbon monoxide, and discloses no recycle of the off-gases, which contain a large proportion of nitrogen.
Nemeth U.S. Pat. No. 3,853,538 discloses a process in which coal or lignite (the emphasis being on the latter) is gasified in a separate gasifier by partial combustion with oxygen, "little or no steam" and very little nitrogen. The gas, said to be primarily carbon monoxide and hydrogen, is desulfurized and introduced into the lower portion, but above the bottom, of a shaft furnace. The off gas is scrubbed and the carbon dioxide is removed. A portion of the cooled and purified gas is introduced into the bottom of the shaft furnace to cool the sponge iron. Another portion is mixed with the reducing gas either in the gasifier or between the gasifier and the desulfurizer.
The use of a separate gasifier complicates the apparatus and increases heat loss.
Galluser U.S. Pat. No. 2,786,747 discloses a process in which ore mixed with coal or coke is introduced into the top of a shaft furnace, while steam is introduced near the midpoint. THe central portion of the furnace is heated electrically to a temperature such that the steam reacts with the coal or coke to produce carbon monoxide and hydrogen and to cause reduction of the iron ore. A temperature of 950.degree. C. is employed, which is below the melting point of carburized iron. A mixture of hydrogen, water, carbon monoxide and carbon dioxide is withdrawn from the top of the furnace. In the upper portion of the furnace they serve to preheat the iron ore and are partially cooled. They are then further cooled, the carbon dioxide is removed and the other gases are recycled to the bottom. Rising through the descending iron ore, they cool it and become heated. They then take part in the reduction of the iron ore.
This process relies on the use of electrical heating, which is expensive.
None of these patents, and no literature of which I am aware, discloses the use of cellulosic material, such as wood waste, municipal trash, or garbage as the fuel in a modern direct reduction process. Charcoal, derived from wood, was, of course, used for centuries in the reduction of iron ore before the introduction of coke.